2. AVANCES PAEI 2017
2.5. Inversiones en Agua Potable y Saneamiento Básico
2.5.3. Inversión
2.5.3.1. Gestión de Proyectos 2017
attempts to access the bus at essentially the same time.
6.7.1 Contention Detection - Contention detection is the recognition of conflicting symbols or bits. The
process of bit-by-bit arbitration allows conflicting frame transmissions to be detected. A node that detects a difference between the symbol or bit it receives and the symbol or bit it is currently transmitting, has detected contention to the transmission of its frame. Only the one frame that wins all conflicts of different symbols and bits with all the other nodes that began transmitting during that frame will not detect contention.
6.7.2 Bit-by-Bit Arbitration - The bit-by-bit arbitration scheme described below settles the conflicts that
occur when multiple nodes attempt to transmit frames simultaneously. This scheme is applied to each symbol / bit of the frame, starting with the SOF symbol and continuing until the end of the frame. Bit-by-bit arbitration is based on the use, at the physical layer, of two values of the bus, called the active state and the passive state. All symbols and bits are encoded on the bus by the physical layer as combinations of active and passive state signals. During simultaneous transmissions of active state and passive state signals on the bus, the resultant state on the bus is always active (active state dominates). If the transmitting node detects a signal state on the bus that is different from the state being transmitted by the node during the header portion of the frame, the node must discontinue the transmit operation prior to the start of the next bit. If the transmitting node detects a signal state on the bus that is different from the state being transmitted by the node during the data portion of the frame (following the header), the process used to cease transmission is implementation specific. Some possible options are:
a. Discontinue the transmit operation prior to the start of the next bit (similar to the action taken if contention is detected during the header phase). If transmission of a message is prematurely terminated on a byte boundary due to unexpected contention detection following the header, care must be taken by receiving nodes not to accept the message as a valid one. There is a 1 in 256 chance that the last byte received is the CRC of the previous bytes and would therefore appear to be a valid message. In this case, a node may need to validate the received message to ensure it is of the correct length.
b. Transmit additional bits (< 8) when contention is detected on a byte boundary. This ensures that a framing error will be received by all other receivers in the case of unexpected contention detection possibly due to noise.
Bus access is granted to nodes sending an active state signal over nodes sending a passive state signal. Figure 21 shows this operation on the physical layer, based on the two forms of modulation allowed (PWM and VPW).
BIT-BY-BIT ARBITRATION ON A PWM BUS
Sending A
┌────┐ ┌──┐ ┌──┐ ┌─┐ ┌─┐ ┌──┐ ┌──┐ ┌─┐
Node A P─┘
└──┘ └─┘ └─┘ └──┘ └──┘ └─┘ └─┘ └─ ─ ─ ─ ─ ─ ─
Contention detected by A ^ Sending A┌────┐ ┌──┐ ┌──┐ ┌─┐ ┌─┐ ┌──┐ ┌──┐ ┌──┐ ┌──┐ ┌─┬─┐
Node B P─┘
└──┘ └─┘ └─┘ └──┘ └──┘ └─┘ └─┘ └─┘ └─┘ └─┴─
Sending A┌────┐ ┌──┐ ┌──┐ ┌─┐ ┌─┐ ┌─┐
Node C P─┘
└──┘ └─┘ └─┘ └──┘ └──┘ └─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─
Contention detected by C ^ Signal on A┌────┐ ┌──┐ ┌──┐ ┌─┐ ┌─┐ ┌──┐ ┌──┐ ┌──┐ ┌──┐ ┌─┬─┐
bus P─┘
└──┘ └─┘ └─┘ └──┘ └──┘ └─┘ └─┘ └─┘ └─┘ └─┴─
│
SOF│
0│
0│
1│
1│
0│
0│
0│
0│
...BIT-BY-BIT ARBITRATION ON A VPW BUS Sending A
┌───────┐ ┌────┐
┌──┐ ┌────┐ ┌──┐
Node A P─┘
└──┘
└────┘ └──┘
└──┘ └── ─ ─ ─ ─ ─ ─
Contention detected by A ^ Sending A┌───────┐ ┌────┐
┌──┐ ┌────┐ ┌────┐
┌──┬─┐
Node B P─┘
└──┘
└────┘ └──┘
└──┘
└────┘ └─┴──
Sending A┌───────┐ ┌────┐
┌──┐ ┌──┐
Node C P─┘
└──┘
└────┘ └──┘ └── ─ ─ ─ ─ ─ ─ ─ ─ ─ ─
Contention detected by C ^ Signal on A┌───────┐ ┌────┐
┌──┐ ┌────┐ ┌────┐
┌──┬─┐
bus P─┘
└──┘
└────┘ └──┘
└──┘
└────┘ └─┴──
The arbitration process begins with the SOF and continues through all the bits of the frames being sent. As each symbol or bit of all the simultaneous frames is transmitted, all of the nodes still transmitting after the last symbol or bit will detect whether any contention is taking place. Nodes that detect contention have, by definition, lost arbitration and will discontinue sending any further symbols or bits. Any remaining nodes will continue to send their symbols / bits until the next contention is detected. The node which obtains sole access to the medium is that which sends its symbols and bits without detecting any contention.
6.7.3 Arbitration Area - The arbitration resolution described above concerns all the symbols and bits
between the SOF and the EOF including, in the case of an in-frame response, all the symbols and bits of the in-frame response. In other words, arbitration acts throughout the frame.
6.7.4 Frame Priority - The bit-by-bit arbitration mechanism allows the implementation of a frame priority
system. When unique frames or, more particularly, when arbitration is resolved down to a single transmitter prior to any data values within the frame, a structure of relative frame priority is inherent.
Should two or more nodes attempt to access the bus within the same frame synchronization window (see IFS in Sections 6.6.1.5 and 6.6.2.7), nondestructive arbitration will occur based on the bit value within each frame. The bus allows two states - active and passive. If a active state bit and a passive state bit are transmitted on the bus simultaneously, then the active state bit will override the passive state bit. Hence, frame arbitration occurs in a bit-wise manner. The result of the arbitration is that the node(s) sending lower priority frame(s) will recognize that they have lost arbitration and will discontinue transmitting before the next bit time. The node transmitting the highest priority frame will continue transmitting uninterrupted.
Based on the bit definitions contained in Section 6.6, the lowest value bytes immediately following the SOF will have the highest priority. Therefore, regardless of the number of bits allocated for "frame prioritization," the numerical value of zero will have the absolutely highest priority (that is, 000 is higher priority than 001 or 111).